Time of flight mass spectrometry (TOFMS) allows the rapid generation of wide range mass spectra. TOFMS is based upon the principle that ions of different mass to charge ratios travel at different velocities such that a bunch of ions accelerated to a specific kinetic energy separates out over a defined distance according to the mass to charge ratio. By detecting the time of arrival of ions at the end of the defined distance, a mass spectrum can be built up.
Most TOFMS operate in so-called cyclic mode, in which successive bunches of ions are accelerated to a kinetic energy, separated in flight according to their mass to charge ratios, and then detected. The complete time spectrum in each cycle is detected and the results added to a histogram.
One of the primary challenges in TOFMS is to maximize the dynamic range of the device. This is primarily constrained by the processing of the signal from the ion detectors: not only must the number of ions arriving be counted, but also the time at which the ions arrive. This data must be obtained and output before the next set of data can be processed.
The earliest TOFMS devices employed analog to digital converters (ADC) to digitize the output of a DC amplifier connected to a collector electrode. The collector electrode in turn received electrons generated by one or more microchannel plate electron multipliers when ions impinged thereon. The output of the ADC was coupled to a charge recorder or oscilloscope and, subsequently, a transient recorder.
Although ADC data acquisition systems do not suffer from the drawbacks of time to digital converters (TDC) (see below), their dynamic range is limited by the non-linearity of the electron multiplier and also by the speed of the ADC itself. Even a fast ADC (<5 ns sampling rate), forming a first part of a transient recorder, has a limited dynamic range, and becomes complex, expensive and problematic at the highest mass accuracies demanded. Also, signal variations on the ADC reduce the mass accuracy of the mass spectrometer.
Time to digital converters (TDC) employ ion counting techniques to allow a mass spectrum to be generated. Here, the impact of a single ion is converted to a first binary value e.g. 1 and the lack of impact is represented as a second binary value (e.g. 0). These data can then be processed via various timers and/or counters.
The advantage of a TDC over the analogue detection technique described above is that the signal output from the electron multiplier in respect of each ion impact is treated identically so that variations in the electron multiplier output are eliminated. There is, however, a limit to the dynamic range of a TDC detector, caused by a so-called dead time associated with ion detection. The dead time occurs immediately following the impact of an individual ion. If a subsequent ion arrives during this dead time, it is not recorded. Thus, at higher ion densities, the total of ions arriving may be significantly more than the number actually detected.
Several techniques have been proposed in recent years to address the problems inherent with ADC and TDC ion detection techniques. WO-A-98/40907 discloses an integrated TDC/ADC data acquisition system for TOFMS. A logarithmic (analogue) amplifier is arranged in parallel with a TDC and also an integrating transient recorder. The TDC can collect data and analyse it in respect of very small ion concentrations whilst the transient recorder is able to collect and analyse data in respect of much higher ion concentrations without saturation. The dynamic range of the data acquisition system overall is thus much larger than that of a traditional TDC without sacrificing sensitivity at lower ion concentrations. However, the problems characteristic of ADC detectors identified above still remain at higher ion concentrations.
Another arrangement is disclosed in an article by Kristo and Enke, in Rev. Sci. Instrum. (1988) vol. 59/3, pages 438-442. The arrangement comprises two channel type electron multipliers in series, together with an intermediate anode. The intermediate anode intercepts the majority of electrons generated by the first multiplier and allows these minority of electrons which are not intercepted to be captured by the second electron multiplier. An analog amplifier generates a first detector output from the anode, and a discriminator and pulse counter generates a second detector output from the second electron multiplier. The outputs of the two detectors are then combined. This technique also suffers from the problems associated with a combined TDC/ADC system.
An alternative approach to the issues of sensitivity and dynamic range is set out in WO-A-98/21742. Here, an array of adjacent but separate equal area anodes is employed, with a separate TDC for each anode. This allows parallel processing of incoming ions, to increase the number of simultaneously arriving ions that are detected and thus to increase the dynamic range. The problem with this, of course, is that increases in the number of detectors increases the cost and, on average, an array of N detectors can only increase the total number of ions detected by a maximum of N times.
To address this, WO-A-99/67801 discloses the use of two anodes of unequal area. This extend the dynamic range of the detector since, with large numbers of a particular ion specie arriving at the detector, the average number of ions detected on the smaller anode is small enough to reduce the effects of saturation. The larger anode, by contrast, can detect ions arriving with a lower concentration without an unacceptable loss of accuracy.
WO-A-99/38190 and WO-A-99/38191 also each disclose a microchannel plate electron multiplier having collection electrodes (anodes) with different surface areas.
Such multiple detector techniques suffer from drawbacks, nevertheless. Firstly, physical cross-talk between the channels is inevitable. Due to the spatial spread of electron clouds created by the electron multipliers, only a part of the cloud may be collected on the smaller anode; similarly partial carry-over of electron clouds from the larger collector can take place. In addition, the close proximity of the anodes causes capacitive coupling between each which in turn increases the likelihood of electronic cross-talk. The multiplier voltage may collapse when very intense ion pulses are received, as is possible in, for example, ICP/MS and GC/MS. This results in reduced sensitivity for subsequent mass peaks. Finally, the ratio of “effective areas” may depend heavily on parameters of the incoming ion beam (which in turn may depend upon space charge, ion source conditions etc.) which leads to a mass dependence upon the ratio. This problem is particularly pronounced in narrow ion beams such as are produced in orthogonal acceleration TOFMS.
U.S. Pat. No. 5,777,326 addresses the last problem outlined above by employing a multitude of similar collectors after a common multiplier. Each collector is connected to a separate TDC channel. Whilst the solution provided by U.S. Pat. No. 5,777,326 does largely remove the mass dependence upon the ratio of anode areas, it fails to address the other problems with this multiple detector arrangement and also extends dynamic range only by a factor equal to the number of channels. Thus, the construction can become complex and even then may not be adequate for certain applications such as gas chromatography/mass spectrometry (GC/MS).
It is an object of the present invention to address the problems of the prior art.
According to a first aspect of the present invention, there is provided an ion detection arrangement for a time-of-flight mass spectrometer comprising: an ion beam splitter arranged to intercept a first part of an incident bunch of ions which has passed through the time-of-flight mass spectrometer, but to allow passage of a second part of that incident bunch of ions; a first detector means arranged to detect ions incident upon the ion beam splitter; and a second detector means arranged to detect those ions which pass through the said ion beam splitter.
The detector of the invention accordingly provides a multiple detector wherein ions that have passed through a TOFMS enter into the detector arrangement through a common entrance window and are then divided by an ion beam splitter such as a conversion dynode or grid. Those ions striking the ion beam splitter generate, in the preferred embodiment, secondary electrons which are detected by a first detector means, whereas those ions passing through the ion beam splitter are detected by a second detector means. The ions are accordingly divided at an early stage in their detection, and the multiple detector arrangement accordingly provides greatly reduced electronic and physical cross-talk between the detectors. The dynamic range is extended without sacrifice of linearity, and better quantitation is available.
Preferably, the ion beam is divided by the ion beam splitter in an unequal proportion such that the vast majority of ions entering the multiple detector arrangement are either intercepted by the ion beam splitter, or, alternatively, the vast majority of ions are not intercepted by the ion beam splitter.
It is preferable that the ion beam is divided into two unequal parts so that one of the detectors continues to operate even when the other is saturated. In preferred embodiments, greater than 90% of the ion beam is allowed to pass through the ion beam splitter which may be, for example, a grid or mesh. Alternatively, less than 10% of the ion beam may pass through the ion beam splitter so that more than 90% is intercepted by it. The latter arrangement is particularly preferred because it is easier to manufacture than a largely transparent grid. Also, the latter arrangement allows secondary electrons which may be generated when the ion beam strikes the beam splitter to be focussed in time of flight as they pass towards the first detector means. Electrons are typically easier to focus than incoming ions because electrons are relatively much lighter and faster than ions so that TOF spreading is correspondingly smaller.
It is preferable that the ion beam splitter is arranged to split the incoming ion beam in such a way that each detector detects ions from multiple points uniformly spread over the width of the incoming ion beam. It is desirable that a representative sample of ions is extracted from across the beam width, not just from one particular point.
According to a second aspect of the present invention, there is provided a method of detecting the time of flight of ions in an ion beam of a time-of-flight mass spectrometer, comprising: directing ions to be detected through the time-of-flight mass spectrometer and toward an ion beam splitter; intercepting a first portion of the ions in the ion beam at the ion beam splitter; allowing passage of a second portion of the ions in the ion beam through the ion beam splitter; detecting ions intercepted by the ion beam splitter with a first detector means; and detecting ions passing through the ion beam splitter with a second detector means.
Further advantageous features are set out in the dependent claims which are appended hereto.